1. Field of the Invention
The present invention relates to an oxide superconducting film and a method of preparing the same.
2. Description of the Related Art
High critical-current oxide superconducting materials which are recently beginning to be put into practical use are expected to be usefully applied to, e.g., a fusion reactor, a magnetically levitated train, a particle accelerator, and a magnetic resonance imaging apparatus (MRI), and some materials are already put into practical use. Y-based superconductors, and the Y-based superconductor having good magnetic characteristics is attracting a great deal of attention as a material which will be put into practical use in the near future.
Examples of a method of preparing a Y-based superconducting film are pulsed laser deposition (PLD), liquid phase epitaxy (LPE), electron beam (EB) processing, and metalorganic, deposition (MOD). Of these methods, non-vacuum, low-cost MOD is being spotlighted in recent years and extensively studied mainly in the United States of America and Japan. It is recently reported that among other MOD methods, MOD using trifluoroacetates (referred to as TFA-MOD hereinafter) can manufacture a superconducting film having good characteristics.
MOD is a method comprising: coating a single-crystal substrate with a chemical solution by spin coating or dip coating, followed by drying the chemical solution to form a gel film, and performing annealing processes twice under normal pressure for the gel film, i.e., calcining and firing, to prepare a superconducting film. In this method, an oxide is formed by decomposing organic materials contained in a precursor by calcining performed in a range of 400° C. to 500° C., and a biaxially oriented structure is formed in the oxide layer by firing performed in a range of 700° C. to 900° C.
MOD has a problem that crystallites are formed after calcining, a disordered oriented structure is formed from these crystallites during firing, and the influence of the phenomenon particularly increases when the thickness is 100 nm or more. To provide a highly oriented structure by this method, it is important to perform rapid heating and rapid cooling within a short period of time so that no crystallites are formed by crystal growth of the thermally decomposed oxide in the calcined film. The rapid heating and rapid cooling are performed by loading and unloading a sample into and from an electric furnace. However, it is difficult to form an even film because the degree of heating of the sample differs between on the central portion and on the edge portion. Therefore, this method requires a large electric furnace capable of precise temperature control. In addition, it is difficult to prepare a superconducting film having good characteristics with high reproducibility because there exist not a little different phases.
As a method that improves MOD described above by which crystallites in the calcined film exert no influence on the fired structure, TFA-MOD is developed. TFA-MOD was first reported by Gupta et al. in 1988. At that time, the purity of solutions was presumably low owing to the effect of starting materials, so TFA-MOD did not provide any particularly outstanding characteristics or reproducibility like other MOD methods. Afterwards, McIntyre et al. improved TFA-MOD, and a superconducting critical current density (JC) exceeding 1 MA/cm2 at 77K and 0 T was realized.
Although TFA-MOD is a type of MOD, crystallites in the calcined film exert no influence on orientation of the fired structure. TEM observation shows that a large number of nanocrystallites exist in the cross-section of the calcined film, but all these nanocrystallites disappear after firing, so a biaxially oriented structure is formed with high reproducibility (T. Araki and I. Hirabayashi, Supercond. Sci. Technol., 16, R71 (2003)). In TFA-MOD, therefore, unlike in normal MOD, carbon which is harmful to superconducting characteristics can be expelled almost completely by calcining continued over 10 hours or more, so a superconducting film having good characteristics can be prepared with high reproducibility (T. Araki, Cryogenics, 41, 675 (2002)). Initially, the growth mechanism during firing was unknown. However, it is recently found that a quasi-liquid network is formed due to mixing of fluorine, and this eliminates crystallites in the calcined film. This reveals in principle that TFA-MOD provides high reproducibility and good characteristics which cannot be realized by any normal MOD (T. Araki et al., J. Appl. Phys., 92, 3318 (2002)).
Unfortunately, TFA-MOD has a problem of difficulty in refining because trifluoroacetate solutions are used. This method finally requires a methanol solution, but trifluoroacetates as carboxylates cause esterification reaction with alcohol, i.e., methanol. If refining is performed in water in order to avoid esterification, hydrogen atoms in water molecules and fluorine atoms in trifluoroacetates form strong hydrogen bonds. Consequently, a large amount, i.e., about 12 wt %, of impurities remain. In a superconducting film prepared from a solution containing impurities, the value of JC is as low as less than 1 MA/cm2 (77K, 0 T).
The solvent-into-gel (SIG) method can address this problem so as to prepare a high-purity solution (T. Araki et al., Supercond. Sci. Technol., 14, L21 (2001); U.S. Pat. No. 6,586,042). In refining of trifluoroacetates, water or alcohols must be used, so molecular trapping through hydrogen bonds is unavoidable. Under the circumstances, based on analysis of the trapping mechanism, it has been found that the SIG method can reduce the impurity amount to about 1/20. As a consequence, the characteristics of the superconducting film improved to JC=7 MA/cm2 (77K, 0 T). Also, 24 YBCO samples having various thicknesses were manufactured by using a high-purity solution prepared by the SIG method and performing optimum calcining in TFA-MOD. Measurement of the JC values of these samples revealed that all the samples had a JC value in a range of 5 to 7 MA/cm2 (77K, 0 T). The very high reproducibility implied that a superconducting film was formed by a growth mechanism entirely different from the conventional one.
In the TFA-MOD method, organic materials are decomposed and, at the same time, a part of an oxide is converted into a fluoride in the first heat treatment (calcining), and a structure oriented on the atomic level is provided by the function of the fluoride in the second heat treatment (firing). Since the reaction is a chemical equilibrium reaction, a small amount of fluoride remains. SIMS analysis from the upper portion of the calcined film toward the substrate surface revealed that one tenth fluorine to Cu, in a molar ratio, remains in the film. SIMS analysis from the upper portion of the fired film toward the substrate surface revealed that about 1/100 to 1/1,000,000 fluorine to Cu, in a molar ratio, remains in the film. The closer the position to the surface is, the higher the fluorine concentration is. This is a phenomenon unique to a chemical equilibrium reaction.
By the use of the SIG method for preparing a high-purity solution described above, it has become possible to prepare high-purity solutions of many lanthanoide (Ln)-based superconductors in addition to Y-based superconductors (YBa2Cu3O7-x), and to realize JC of 3 MA/cm2 (77K, 0 T) for Gd-, Er-, Dy-, and Tm-based superconducting films. However, no good characteristics can be realized for La-, Nd-, and Sm-based superconducting films (having TC values of 100K, 96K, and 94K, respectively, in the bulk), although they are practically important. This is because an esterification reaction occurs under the initial conditions of solution synthesis. A Sm-based superconducting film prepared by solution synthesis under a conditional range over which no esterification reaction occurred exhibited TC of 84K at the best (U.S. Pat. No. 6,586,042 described above).
The esterification reaction described above is closely related to the atomic radii of lanthanide elements. There is a tendency that the smaller the atomic number of a lanthanide element is, the larger the atomic radius is. If the atomic radius of the central metal element of a trifluoroacetate is large, alcohol readily approaches the central metal element to allow easy occurrence of the esterification reaction. Accordingly, compared to Gd-, Er-, Dy-, and Tm-based materials, La-, Nd-, and Sm-based materials increase the possibility that alcohol approaches the central metal even though the atomic radius increases only slightly, so the esterification reaction readily occurs.
To suppress the esterification reaction, it is presumably effective to increase the molecular weight of carboxylate or alcohol. This is so probably because if high-molecular-weight carboxylate or alcohol is used, the esterification reaction conditions shift to higher temperatures or lower pressures, and as a consequence the esterification reaction can be suppressed.
However, to increase the molecular weight of carboxylate for suppressing the esterification reaction would increase the possibility that carbon remains in the film during calcining and deteriorates the superconducting characteristics. Also, when the SIG method was introduced to TFA-MOD, a mechanism by which carbon atoms harmful to the superconducting characteristics were expelled outside was unknown. Therefore, all the experiments using carboxylates having long chains provided significantly deteriorated characteristics.
On the other hand, it was assumed that alcohol did not increase the residual carbon amount because alcohol was used as a solvent and volatilized during coating. It has been tried to suppress the esterification reaction by using substitute solvents such as ethanol, 1-propanol, 2-propanol, and 1-butanol, instead of methanol. When ethanol was used, it was possible to suppress the esterification reaction and prepare high-purity ethanol solutions for all of La-, Nd-, and Sm-based materials. Similarly, the esterification reaction was suppressed when alcohols having larger molecular weights were used. However, when a Sm- or Y-based superconducting film was manufactured by preparing an ethanol solution, propanol solution, or butanol solution, coating the solution to form a gel film, and calcining and firing the gel film, the characteristics of the resultant superconducting film were significantly deteriorated. This is presumably because ethanol, propanol or butanol is inferior to methanol in volatility, so that a slight amount of alcohol remains in the gel film during coating, and this residual alcohol leaves residual carbon during firing, which deteriorates the superconducting characteristics.
Of La-, Nd-, and Sm-based superconducting films prepared by the method of suppressing esterification by using ethanol, only the Sm-based superconducting film exhibited superconductivity at a liquid nitrogen temperature, specifically, TC of 88.4K and JC of 0.60 MA/cm2 (77K, 0 T). Even in this example, however, TC was much smaller than TC=94K of a bulk Sm-based superconductor, and was also smaller than TC=91K of a bulk Y-based superconductor. Accordingly, even this superconducting film was unsatisfactory to be put into practical use.